This invention relates to a photovoltaic device adapted to provide electrical energy in response to irradiation and, more particularly, to a multi-layer semiconductor photovoltaic device having improved photoelectric conversion efficiency.
The phenomenon of converting light energy into electrical energy has long been known. Photoelectric cells adapted to perform this function have been in existence for many years. However, these devices have not been readily utilized because they suffer from low photoelectric conversion efficiency and from low photovoltaic energy density. That is, the wattage per square centimeter which is produced in response to incident light is undesirably low. Consequently, the electrical energy which can be produced by such photoelectric cells is undesirably low and is not suited for many applications.
As is known, the spectrum of radiant energy having the very high frequencies so as to be classified as light, such as the solar spectrum, is comprised of various frequencies, or wave lengths. It is also known that if the energy included in incident light exceeds the band gap energy of certain semiconductor materials, the material will be sufficiently excited so that an electron is emitted, or at least freed so as to support an electric current. That is, when the photon energy hf, wherein h is Planck's constant and f is the frequency of the incident light, exceeds the band gap energy E.sub.g, electrons in the valance band of the semiconductor material are excited into the conduction band so as to form a hole-electron pair. These charge carriers are adapted to provide a current.
Most photoelectric cells heretofore known are formed of only two regions, or layers, constituted by P-type material adjacent a N-type material, thereby defining a PN junction therebetween. Electrodes are attached to the respective P and N materials so as to supply an output voltage and an output current when the photoelectric cell is irradiated. In accordance with the known photoelectric phenomenon, incident light whose energy is hf excites an electron in the valence band into the conduction band, thereby forming the aforenoted electron-hole pair. That is, the electron now in the conduction band is paired with a hole in the valence band from whence the electron came. The electric field across the PN junction separates the electron-hole pair so that the electron is collected in the N-type region and the hole is collected in the P-type region. Because of this charge migration, the Fermi level is not continuous across the PN junction. Rather, there is a difference in the Fermi level between the P-type region and the N-type region, resulting in a photovoltaic output voltage V.sub.o which is proportional to this difference. This output voltage can be utilized by connecting a suitable load to the photoelectric cell.
The semiconductor photoelectric cell is subjected to various losses therein, some of which are capable of being diminished, while others are theoretically impossible to reduce. These losses are as follows:
Optical loss: This is the loss in the light energy caused by reflection at the surface of the photoelectric cell and by transmission of light through the cell without absorption.
Quantum loss (long wavelength): This is the loss in photon energy wherein hf is less than the band gap energy E.sub.g.
Quantum loss (short wavelength): This is the loss in the photon energy which is much higher than the band gap energy and is converted to heat. It is considered that the conversion of photon energy to heat and not to electrical energy is a loss.
Collection loss: This is the loss due to the recombination of minority carriers which, after being diffused across the PN junction are recombined with majority carriers. It is considered that, if not for this recombination, the diffusion of minority carriers can be derived as a useful current.
Potential factor loss: This is the loss caused by the drop in potential energy of a charge carrier when the carrier crosses the PN junction.
Impedance factor loss: This is the loss in energy caused by the internal resistance of the photoelectric cell and the leakage current across the PN junction.
Although most known semiconductor photoelectric cells are formed of only two layers, a multiple junction photoelectric cell has been described by M. Wolf in Proceedings of IRE, Vol. 48, No. 7, p. 1246. A multi-layer semiconductor device also is described in U.S. Pat. No. 3,046,459 and in U.S. Pat. No. 3,186,873. However, some of the layers of these multi-layer devices exhibit long diffusion lengths. Consequently, it is expected that the collection loss in these multi-layer devices is high, resulting in relatively low photoelectric efficiency and low photovoltaic energy density. Accordingly, these prior art devices are not well suited for many applications as a semiconductor photoelectric cell.